Foundries use refractory granules such as sand, which is bound together with a resin binder, to form shell molds and cores used for casting metal and other molten materials. Typically, a minor proportion of uncured resin and curing agent are combined with new and/or reclaimed refractory particulate material (e.g., sand). The resulting composition is mulled or kneaded at elevated temperature, such that the resin is uniformly dispersed (coated) over the refractory particulate material. The resin-coated refractory particulate material, or molding composition, is then placed onto a heated pattern, which is used to form the refractory particulate material into a desired shell or core shape. The heat from the pattern, which is generally later accompanied by external heat from the opposite or outer side of the refractory particulate material layer, is used to set or cure the resin binder and provide a rigid, cured refractory particulate material mold. A shell mold may be formed by gluing two refractory particulate material mold halves, prepared in this manner, together to form a cavity suitable for retaining molten metal (e.g., iron or steel) in metal casting operations. A core mold is optionally placed within a shell mold, if a hollow metal casting is desired.
Various agents, such as mold lubricants (e.g., calcium stearate), may be added to the resin binder to improve the flow/packing characteristics of the molding composition, resulting in higher density and strength of the cured mold. Clay is also sometimes added to sand and incorporated into the molding composition to improve the finish of the cast metal.
In preparing cast metal articles, after molten metal is introduced into the mold cavity, the metal is cooled as its heat is transferred to the mold, causing the resin binder to break down. This allows for a clean and efficient removal of the remaining refractory particulate material (e.g., sand or sand/clay blend) from the cast metal article. In some cases, particularly when the cast metal has a low melting point, mechanical force may be needed to break the mold and/or calcination may be needed to sufficiently thermally degrade the binder in the mold.
After separation of the refractory particulate material from the cast metal article, it is advantageously subjected to thermal reclamation, whereby the organic materials of the binder are more completely volatilized (i.e., burned off). This allows for reuse of the refractory particulate material after a number of cycles of preparing molding compositions and casting metals as described above. The ability to thermally reclaim the refractory particulate material, however, has traditionally been limited by the gradual reduction in quality, and particularly the strength characteristics, of the molds made from the thermally reclaimed refractory particulate material, from one thermal reclamation cycle to the next. The standard industry practice of addressing this problem, prior to each thermal reclamation cycle, is to improve the quality of the refractory particulate material by removing clay materials, diluting it with fresh refractory particulate material, and/or washing it to remove calcium.
Various issues associated with refractory particulate material reclamation are discussed in the art. For example, Roeth G., et al GIESSEREIFORSCHUNG, 50(1): 10-24 (1998) characterizes reclaimed molding sands in terms of a number of selected criteria. Oehlerking, T. GEISSEREI, 80(21): 721-8 (1993) evaluates the usability of reclaimed molding sand as a function mixing ratios and other parameters. Granlund, M. et al. TRANSACTIONS OF THE AMERICAN FOUNDRYMEN'S SOCIETY, 91:101-8 (1983) describes the benefits of calcination in thermal reclamation.
Phenolic resins and especially phenol-formaldehyde resins such as novolacs have gained acceptance as binders in the production of the shell and core molds described above due to their excellent performance in this demanding service. In many cases, novolac resin that is a solid at ambient temperature (e.g., novolac flake) is melted onto the heated refractory particulate material to provide the molding composition. Also, particularly in the case of a novolac, a polyfunctional curing agent such as hexamethylenetetramine (hereinafter “hexamine”) is required to cross-link and harden the resin. A sufficient quantity of hexamine is required to achieve a suitable tensile strength of the mold for metal casting.
While hexamine can convert thermoplastic novolac resins into desired thermosetting resins, hexamine is known to emit pollutant/contaminant gases such as ammonia, amines, and formaldehyde as a result of these cross-linking reactions during the refractory particulate material coating and molding operations, as well as during pyrolysis of the iron or steel casting. Smoke and odors resulting from the use of hexamine are also significant concerns. Moreover ammonia and amines that remain in the molds can corrode the cast metal products, as well as lead to mechanical failure and defects such as pinholes or blow holes, due to the volatilization of these components.
To offset some of the above-noted disadvantages associated with the use of hexamine, some resin binder systems incorporate a thermosetting phenolic resole, together with the novolac, in order to reduce the amount of hexamine required for curing. Phenolic resole resins exhibit slower curing characteristics and are more difficult to control in terms of their degree of polymerization, when compared to purely novolac/hexamine systems. Additives are therefore generally used to catalyze and better control the cure of phenolic resole resins. Such additives are described, for example, in Japanese Patent Publication Nos. 53-58430 and 54-28357 and include hydroxides, oxides of magnesium, zinc and barium, bisphenol S, catechol, reactive phenols such as resorcinol, and acids such as salicylic acid.
Various phenolic resin binder compositions are described in the art. For example, U.S. Pat. No. 4,460,717 describes a phenolic resin comprising an aromatic ring compound, which purportedly allows for greater ease of removal of the mold from the cast metal, when this metal has a lower melting temperature than iron.
U.S. Pat. No. 4,426,484 describes phenolic resole resin binders having specified cure characteristics that are used to coat sand and prepare molding materials.
U.S. Pat. No. 4,252,700 describes the use of a lubricant-containing solid resole resin, as a curing agent for a novolac resin to provide faster curing, increase cross-link density, and achieve various other properties in binding sand used to form molds.
U.S. Pat. Nos. 4,397,967 and 4,403,076 describe novolac resins having improved cure speed, which are used to coat sand for the preparation of molds and cores having good tensile strength properties.
The art has not satisfactorily addressed the problems described above that prevent the reuse of refractory particulate material over a significant number of thermal reclamation cycles, in molding compositions comprising a phenolic resin binder. Accordingly, there remains a need for phenolic resin binder systems that allow refractory particulate material to be reused over multiple thermal reclamation cycles, without suffering from a loss in tensile strength and/or higher crumbling and cracking tendency of the resulting molds over time, especially when a sand/clay blend is used. Ideally, such binder systems also should have low emissions (including volatile organic carbon (VOC), ammonia, amines, smoke, and odors), a low tendency to form defects in the cast metal articles, and consequently a low requirement for the use of hexamine as a hardening agent to compensate for lost tensile strength. The binder systems should have various properties, discussed hereinafter, that are well suited to the formation of molding compositions. For example, the binders should be able to hold the shape of the mold as it is cured, without the separation of partially-cured or tacky molding composition (a phenomenon known as “peelback”). The binder systems should also provide good finishing characteristics of cast metal articles prepared from the molds, whether or not clay is incorporated into the molding composition.